Electro-luminescent (EL) lamps are used often for backlighting. The advantages of the EL lamp include its thinness, its light weight and flexibility. The disadvantage of the use of an EL lamp is that these phosphor based lamps dim with age as the phosphor decays. To overcome this effect, the voltage or frequency applied to the lamp can be increased to cause the phosphor to light be brighter, thus compensating for the reduction in light emission from the decay.
There are several approaches for compensating for the phosphor decay. One approach is to use a constant power driver circuit to compensate life. As the EL lamp decays, the reduced capacitance result in lower deriving current and deriving power. A constant power deriving circuit could increase output voltage as the lamp ages, but it is not possible to maintain constant output brightness, since there is no measurable parameter to indicate how much the lamp has aged.
Automatic brightness control systems include optical feedback using a monitoring photocell which is currently the most effective method to measure and control the brightness output. Based on a reading, the voltage and frequency supplied to the EL lamp are adjusted. In this way, the lamp brightness output can be controlled and maintain constant. This method has many inherent problems: it is expensive, it is affected by surrounding ambient light, it needs a response calibration based on the specific application and the optical monitoring device must be appropriately mounted close to the EL lamp.
Another method to compensate for reduced brightness of an EL lamp is to configure a digital circuit which includes a microprocessor, a timer and ROM. A table in ROM represents the relationship between the appropriate derive voltage and on-time to produce the constant brightness required. The microprocessor will track the total on-time of EL lamp and select appropriate data from table from the deriving EL lamp. However, as the EL lamp changed or expired, the timer should be reset. This open loop control system is not effective for an EL lamp.
Thus, there is a need to provide a simple, measurable parameter to indicate the age of the EL lamp and use this parameter to maintain the required constant brightness.